1. Field of the Invention
The present invention relates to a laser annealing apparatus, and more particularly, to a laser annealing apparatus including an adjustment device for tuning the shape of linear laser beams.
2. Description of the Related Art
In order to control electrical and physical characteristics of a semiconductor used as a channel in a thin film transistor, amorphous silicon, which is a component of the semiconductor, needs to be crystallized into polycrystalline silicon by subjecting amorphous silicon to a thermal treatment process. When a silicon thin film such as an amorphous silicon thin film or a monocrystalline silicon thin film is deposited as a semiconductor layer at a low temperature which is appropriate for a glass substrate, the deposited silicon thin film has a relatively low crystallinity.
As a method for improving the low crystallinity of the amorphous silicon thin film (hereinafter includes the monocrystalline silicon thin film), a laser annealing process in which a laser beam is projected onto the amorphous silicon thin film, is currently used. As a result of the instantaneous heating by the laser annealing process, crystallization of the amorphous silicon thin film is induced.
In the laser annealing process, the crystallization occurs due to the instantaneous heating. Thus, the laser annealing process has an advantage in that a glass substrate that is used as a substrate of an organic light-emitting diode (OLED) or a liquid crystal display (LCD), experiences less damage in comparison to a general heating process such as a furnace annealing process or a rapid thermal annealing process. The laser annealing process also has an advantage in that the electrical characteristics of crystallized polycrystalline silicon are excellent.
The laser annealing process is performed by generating a linear laser beam having a linear cross-sectional shape obtained by expanding or homogenizing an oscillated laser beam through a beam expander or a beam homogenizer, and focusing the expanded or homogenized laser beam through a cylindrical lens to form a single linear laser beam, and by projecting the linear laser beam onto the amorphous silicon thin film, and making the linear laser beam to scan the surface of the silicon thin film in a fixed scan pitch. In general, an excimer (exited dimer) laser beam which is a pulse laser, is used as a laser beam for the laser annealing process.
In a contemporary excimer laser annealing apparatus, a rectangular raw beam having a rectangular cross-sectional shape passes through an optical system including a beam homogenizer, a reflector, and a focusing lens to be divided into a plurality of linear laser beams that are subsequently overlapped and homogenized to finally form a single linear laser beam having a beam width (BW) of 0.4 mm and a beam length (BL) of 270 mm. That is, the cross-sectional shape of the linear laser beam is linear.
After the linear laser beam output from optical system is focused on a substrate to be annealed, the linear laser beam moves in a direction perpendicular to the lengthwise direction of the linear laser beam with a fixed scan pitch. Alternatively, a stage for holding the substrate may move in a direction opposite to the direction in which the linear laser beam moves. As a result, an amorphous silicon thin film on the substrate is annealed. In this case, regions where laser annealing is repeatedly performed, appear on the amorphous silicon thin film in accordance with a scan pitch of the linear laser beam, and these regions may create stripy stains due to an energy distribution profile of the linear laser beam across the widthwise direction of the linear laser beam. In other words, when the linear laser beam scans the irradiated surface of the amorphous silicon thin film by moving step-by-step with a fixed scan pitch in a direction perpendicular to the lengthwise direction of the linear laser beam, the region irradiated by the linear laser beam in a current scanning step will be overlapped by the region irradiated by the linear laser beam in a subsequent scanning step. The overlapped region may receive a laser energy which is different than that of other region. As a result, the amorphous silicon thin film will receive uneven laser energy and a stripy pattern will be formed on the amorphous silicon thin film.
In order to avoid the stripy pattern, the energy distribution of the linear laser beam must be controlled to have a desired profile. If the desired energy distribution profile of the linear laser beam can not be obtained, however, the optical system has to be modified and reset. Furthermore, if the desired energy distribution profile of the linear laser beam changes due to a reason, for example, a thickness of the substrate or a surface to be processed (the amorphous silicon thin film) changes during an annealing process, the contemporary excimer laser annealing apparatus has to be manually modified and the optical system have to be reset.